This invention relates to the application of electrodes to ceramic piezoelectric transducers.
Piezoelectric transducers must be provided with electrodes which have good electrical coupling to the piezoelectric transducer surface in order to produce the maximum possible deformation of the transducer in response to applied electric potentials. Ceramic piezoelectric transducers of the type used in ink jet systems, such as shear mode transducers which are in the form of a thin plate or layer of ceramic material like lead zirconium titanate (PZT), must be provided with a series of closely spaced electrodes on one or both surfaces. Heretofore, because of the granular surface structure of such ceramic materials, it has been necessary to apply electrodes to those surfaces by evaporating or sputtering thin layers of metal, such as copper or gold, onto the surfaces in order to provide high capacitive coupling, before subjecting the piezoelectric layer to an electric field to pole the piezoelectric material.
Conventional poling techniques for such piezoelectric transducers require that the electrode metal be applied in that manner to both surfaces before the piezoelectric material is polarized in order to obtain maximum polarization. Thereafter the previously applied electrode metal may be patterned by photolithographic etching techniques to provide electrodes at the desired locations on the opposite surfaces of the piezoelectric layer.
Recently, however, poling techniques for polarizing piezoelectric transducers have been developed which do not require prior application of metal layers to the transducer surfaces. For example, in the copending Moynihan et al. application Ser. No. 08/460,393 filed Jun. 2, 1995 now U.S. Pat. No. 5,605659 and its parent application Ser. No. 08/406,297 filed Mar. 17, 1995, the disclosures of which are incorporated herein by reference, pressure poling and corona poling techniques are disclosed which eliminate the need for the presence of electrodes on the surfaces of the transducer material for poling of the material.
Furthermore, since the application of electrode material to the surfaces of piezoelectric materials by conventional evaporation or sputtering techniques is a complex procedure and represents a substantial portion of the cost of producing electroded piezoelectric transducers, it would be advantageous to provide a way of electroding piezoelectric transducers without requiring such complex and expensive steps. In addition, conventional piezoelectric transducers have electrodes which cannot be extended beyond the area of a surface of the transducer and thus require connection to remote actuating circuits by separate electrical leads, adding to the complexity and expense of manufacture.
Accordingly, it is an object of the present invention to provide a method for electroding piezoelectric transducers which overcomes the disadvantages of the prior art.
Another object of the invention is to provide electroded piezoelectric transducers and ink jet printheads incorporating such transducers which do not require the complex and expensive electroding procedures of the prior art.
A further object of the invention is to provide a method for electroding piezoelectric transducers which eliminates the need for wiring connections at a surface of the piezoelectric transducer.
These and other objects of the invention are attained by applying a pattern of deformable electrode material corresponding to the desired electrode pattern to a surface of the piezoelectric material under sufficient pressure to assure uniform and high electrical coupling between the conductors and the surface of the piezoelectric material. In one embodiment the pattern of electrode conductors is formed on one surface of a dielectric film and the electrode pattern is bonded to the surface of the ceramic piezoelectric plate with sufficient uniformly applied pressure to conform the electrode material to granular irregularities in the surface of the piezoelectric plate so as to assure conductive contact between the electrode pattern and localized asperities in the surface of the piezoelectric plate. For this purpose the pressure may be applied through an elastomeric or viscoplastic layer.
Preferably, the dielectric film and electrode film are bonded to the surface of the piezoelectric layer with a thin layer of adhesive material, such as an epoxy resin. Because the adhesive layer has a large effect on the capacitive coupling between the electrode film and the piezoelectric layer, the adhesive layer should be as thin as possible while still assuring good bonding. If desired, the bonding material may incorporate fine particles of high dielectric or conductive material of appropriate size and distribution to provide improved capacitive coupling or localized conduction between the surface of the piezoelectric plate and the electrode without producing surface conduction along the surface of the piezoelectric plate. Furthermore, the dielectric film may be removed from the electrodes, if desired, after the electrodes have been bonded to the piezoelectric plate by making the bond between the electrode material and the piezoelectric layer stronger than that between the dielectric layer and the electrode material.
In order to permit connection of the electrodes on the surface of the piezoelectric plate directly to a remote source of driving voltage, the dielectric layer may extend beyond the edge of the piezoelectric plate and may contain conductors formed of the electrode material which connect the electrode pattern on the piezoelectric layer to a remote driving component, such as for example, an integrated circuit or drive chip which may be affixed to the dielectric film and may contain driving elements for the electrodes located at selected portions of the piezoelectric layer.
For use in an ink jet head, the electroded piezoelectric transducer may be affixed to a chamber plate which is formed with ink pressure chambers corresponding to the electroding applied to one surface of the piezoelectric transducer and the chamber plate may include passages leading from the pressure chambers to drop ejecting orifices. In certain embodiments, the ink jet head may include a chamber plate made of carbon as described in the above-cited Moynihan et al. applications. Moreover, the chamber plate may have ink chambers formed on both sides and piezoelectric transducers with affixed electrodes may be mounted on both sides of the carbon plate with the electrodes positioned in accordance with the chambers formed in the plate.